Jack Vance is one of my favourite authors, and a few of his books are easily among my top 5 favourite books (assuming, as I do, that I'm allowed at least one top-5 book lists per author).
The book Dying Earth is an early (possibly his first novel, but I'm no literary historian) Vance fantasy novel that, like a lot of fantasy that came before the Tolkien-craze that more or less persists even today, knows nothing of our latter day tropes and expectations. It's a brand of fantasy that borders on science fiction, but not in the way that you'd call it "science fantasy".
The magic system in this book, inarguably, is an influence on the magic in D&D: wizards struggle to recall spells that leave almost physically leave their minds upon casting, and their powers exhaust them when over-used. If that's not enough, here's some trivia for fellow D&D nerds: Vecna is an intentional anagram of Vance.
Wizardry in this book looks a lot like mad science, and the world is populated by planar travelers, cunning witchcraft and enchantment, and a past so far in the future that it betrays the laws of fantasy and yet ignores the laws of sci fi. I think of the book as the perfect, bizarre blend of two great RPGs: D&D and DeadEarth (not to be confused with the "Dying Earth" RPG, which is indeed is based directly on this book); it's got the quests and lore of a good D&D game with the despotism and depravity of DeadEarth.
But Vance's real strength, at least to me, is the culture he builds in his stories. He manages to explain just enough about the world you're spending time in, whilst leaving just enough a mystery to let your mind run with your own explanation of how the world works. His characters are alive and dedicated to their absurd reality just as much as you and I are dedicated to our absurd reality.
Technically, this is a series of short stories connected, such as they are, by a few characters who make appearances in a few stories throughout. It's sort of a shared universe setup, and works perfectly in tricking you into thinking you're reading a novel rather than a collection.
By my estimation, this is a perfect book. It's maybe a little weird, if you're expecting either traditional fantasy or traditional sci fi, but if you're open to the unexpected or if you're curious about early influences on Dungeons & Dragons, then you must read this. And anyway, you should read it, because Jack Vance is a truly amazing author.